Historically, coins have been recognized and differentiated by their appearance including coin dimensions, edge thickness, shape, color, serrations and engravings on obverse and reverse sides of the coin.
Coins may also be differentiated by their chemical composition. Vending machine coin acceptors generally rely on electromagnetic differences between coin materials to differentiate between types of coins by generating an electromagnetic signal as a coin passes through a pair of energized coils. The electromagnetic signal is analysed through a decomposition of the sinusoidal wave form of the electromagnetic signal into elements which may be related to diameter, thickness, and material magnetic characteristics. Many coin acceptors are unable to accurately differentiate between coins of different countries when the coins are made of the same alloy or the same family of alloys, such as bronze or brass. Further, many coin acceptors are unable to recognize plated steel coins.